1. Field of the Invention
The present invention relates to air-fuel ratio control apparatus of internal combustion engine.
2. Related Background Art
For purifying exhaust gas from the internal combustion engine, an exhaust purifying catalyst (three-way catalyst) is placed on the exhaust path, an air-fuel ratio sensor is located on the exhaust path to detect the air-fuel ratio (A/F), and feedback control is implemented so as to control an air-fuel mixture toward the stoichiometric air-fuel ratio, thereby simultaneously decreasing nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC). The purification efficiency of exhaust gas emitted from the internal combustion engine is further increased effectively by carrying out the foregoing feedback control with accuracy.
The inventors found that further improvement was made in the purification efficiency of NOx, CO, and HC, noting the oxygen occlusion function of the exhaust purifying catalyst. An object of the present invention is to provide air-fuel ratio control apparatus of internal combustion engine with superior exhaust purifying characteristics, while effectively making use of the oxygen occlusion function of the exhaust purifying catalyst.
The air-fuel ratio control apparatus of the internal combustion engine according to the present invention was accomplished based on the above finding and comprises oxygen storage amount estimating means, downstream exhaust air-fuel ratio detecting means, maximum oxygen storage amount estimating means, and air-fuel ratio target setting means. The oxygen storage amount estimating means estimates an oxygen storage amount of an exhaust purifying catalyst, based on a history of an oxygen adsorption/desorption amount of the exhaust purifying catalyst located on an exhaust path. The downstream exhaust air-fuel ratio detecting means is located downstream of the exhaust purifying catalyst and detects an exhaust air-fuel ratio downstream of the exhaust purifying catalyst. The maximum oxygen storage amount estimating means estimates a maximum oxygen storage amount, based on an oxygen storage amount estimate when the exhaust air-fuel ratio detected is a predetermined air-fuel ratio. This permits effective utilization of the oxygen occlusion capability of the exhaust purifying catalyst and improvement in the exhaust purification performance.
Further, the apparatus may also be configured to set a target of amount of oxygen occluded in the exhaust purifying catalyst (oxygen storage amount), to approximately half of a maximum storage amount of oxygen that the exhaust purifying catalyst can occlude (maximum oxygen storage amount) and control the air-fuel ratio so as to bring the oxygen storage amount toward this target. In this case, if the maximum oxygen storage amount is a fixed value (or a corrected value from the fixed value), there will also appear situations that do not always reflect the real maximum oxygen storage amount and there can also appear circumstances failing to utilize the performance of the exhaust purifying catalyst at its maximum. Therefore, improvement may be made so as to achieve further advanced purification characteristics. The inventors also investigated it and invented more preferable apparatus as described below.
In a preferable aspect, the oxygen storage amount estimating means estimates the oxygen storage amount, based on a quantity of fuel supplied into a cylinder, which is predicted according to a fuel behavior model taking account of fuel attaching onto an internal surface of an intake path of the internal combustion engine. In a further preferable aspect, the apparatus further comprises an upstream air-fuel ratio sensor located upstream of the exhaust purifying catalyst on the exhaust path, for detecting an exhaust air-fuel ratio upstream of the exhaust purifying catalyst, and the oxygen storage amount estimating means estimates the oxygen storage amount in consideration of a response delay of the upstream air-fuel ratio sensor.
In another preferable aspect, the oxygen storage amount estimating means estimates the oxygen storage amount, based on a passing gas flow rate of gas passing through the exhaust purifying catalyst. In still another preferable aspect, the oxygen storage amount estimating means estimates the oxygen storage amount, based on the magnitude of change of the air-fuel ratio of the internal combustion engine.
In a further preferable aspect, the maximum oxygen storage amount estimating means further corrects the maximum oxygen storage amount estimated, based on a passing gas flow rate of gas passing through the exhaust purifying catalyst. In another preferable aspect, the maximum oxygen storage amount estimating means further corrects the maximum oxygen storage amount estimated, based on the magnitude of change of the controlled air-fuel ratio.
In a further preferable aspect, the oxygen storage amount estimating means estimates the oxygen storage amount, based on composition of exhaust gas flowing into the exhaust purifying catalyst, which is predicted according to a combustion model. In a more preferable aspect herein, the combustion model is modified based on quality of fuel. In another more preferable aspect, the combustion model is modified according to an operational state of the internal combustion engine.
In another preferable aspect, a result of detection by the downstream exhaust air-fuel ratio detecting means is corrected based on the composition of exhaust gas flowing into the exhaust purifying catalyst. In a more preferable aspect herein, the composition of the exhaust gas is predicted according to a combustion model.
In another preferable aspect, the apparatus further comprises abnormality determining means for determining whether the oxygen storage amount estimated is abnormal, based on the oxygen storage amount estimated and the downstream exhaust air-fuel ratio detected. In a more preferable aspect herein, when determining that the oxygen storage amount estimated is abnormal, the abnormality determining means deletes the old history of the oxygen storage amount and initiates new estimation of the oxygen storage amount. In a further more preferable aspect, when determining that the oxygen storage amount estimated is abnormal, the abnormality determining means modifies an oxygen occlusion amount estimation model.
In another preferable aspect, the apparatus further comprises abnormality determining means for determining that the maximum oxygen storage amount estimated is abnormal, when the maximum oxygen storage amount estimated by the maximum oxygen storage amount estimating means exceeds a predetermined value.
In another preferable aspect, the apparatus further comprises catalyst deterioration detecting means for detecting a degree of deterioration of the exhaust purifying catalyst, based on a change slope of the exhaust air-fuel ratio detected by the downstream exhaust air-fuel ratio detecting means.
In another preferable aspect, a blow-by occurrence oxygen storage amount, at which a blow-by phenomenon not utilizing the oxygen occlusion capability of the exhaust purifying catalyst fully occurs, is preliminarily set and the oxygen storage amount is calibrated based on the blow-by occurrence oxygen storage amount upon occurrence of the blow-by phenomenon.
In another preferable aspect, the apparatus further comprises storage amount target setting means for setting an oxygen storage amount target, based on the maximum oxygen storage estimate estimated, and the storage amount target setting means applies forced oscillation to the oxygen storage amount target.
In another preferable aspect, the air-fuel ratio target setting means sets the target so as to gradually increase the amplitude of the controlled air-fuel ratio. In another preferable aspect, the air-fuel ratio target setting means sets the target so as to gradually prolong the cycle of the controlled air-fuel ratio.
In another preferable aspect, the apparatus further comprises storage amount target setting means for setting an oxygen storage amount target, and the storage amount target setting means sets a plurality of targets between a minimum and a maximum of the oxygen storage amount and sets a control target by switching among the targets.
In another preferable aspect, the air-fuel ratio target setting means sets the target of the controlled air-fuel ratio in a lean region immediately after cold starting or during low-load operation of the internal combustion engine. In another preferable aspect, the air-fuel ratio target setting means sets the target of the controlled air-fuel ratio in a rich region during high-load operation of the internal combustion engine. In still another preferable aspect, the air-fuel ratio target setting means sets an operation period of setting the target of the controlled air-fuel ratio in a rich region, before execution of cutting fuel into the internal combustion engine or before a stop of the internal combustion engine.
In another preferable aspect, the air-fuel ratio target setting means sets a gain of air-fuel ratio feedback control, based on the oxygen storage amount estimated by the oxygen storage amount estimating means or based on the maximum oxygen storage amount estimated by the maximum oxygen storage amount estimating means.
In a further preferable aspect, the apparatus further comprises catalyst deterioration detecting means for detecting deterioration of the exhaust purifying catalyst, based on output of the downstream exhaust air-fuel ratio detecting means provided when the controlled air-fuel ratio is controlled so that the oxygen adsorption/desorption amount goes into a range of an instantaneously occludable oxygen amount or instantaneously releasable oxygen amount in a nondeteriorated state of the exhaust purifying catalyst.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.